Rework association between series and documents

Instead of linking series with editions, we now link directly to the
document. This removes the need to copy across the association with
each new edition and greatly simplifies the code in general. We no
longerr list series documents grouped by type on frontend - this added
little value to the user.
DRY things up: kill off DocumentSeriesFinder

SluggedModelFinder does exactly the same work
Rework Importer to work with new doc series assoc

I'm not entirely happy with the way more code is being chucked into
Import#perform, but without a wholesale refactor of the way the import
engine works, I can't see how else to handle this.
Remove fields for doc series from editon form
WIP - FEATURE REWRITE
Admin::EditionFilter page size can be overridden
Introduce tabbed navigation for document series
Introduce Whitehall::DocumentSimpleSearch

This is to allow us to do quicker keyword-based searching of documents
than with the full-blown Admin::EditionFilterer. For now this simply
uses Mysql LIKE queries under the hood. Eventually we want to have a
separate admin elastic search index that indexes non-published
content.

This commit also adds some inverse relationships to Document->Edition
associations to avoid unnecessary queries.
JavaScript for DocumentFinder
Add admin/editions#search action

I originally set off down the road of replicating what is done on
front-end with the document filterer (i.e. use presenters/delegators
to wrap search results and convert them to a suitable JSON
representation), but using JBuilder provides a much simpler and
lighter-weight way of doing the same thing.
Wire in the form for the document finder
Move finder results out of form

The results will include a form for adding documents to the series,
so we don't want them embedded within the main search form.
Add usesful attributes to finder results
Only load the most recent edition
Allow documents to be added to series
Add remove from document series
Only show 10 most recent results
Make sure DOM IDs are unique
More feedback on add/remove from document series
Include document count in series tabbed nav